The present invention relates to an optimum stop control system for turning off a building's temperature control system an amount of lead time prior to the end of occupancy of the building in order to conserve energy or starting such temperature control system an optimum amount of lead time prior to the beginning of occupancy and, more particularly, to such a system wherein the amount of lead time is based both upon the drift in temperature within the building during a period of time when the temperature control system is off or on depending on whether the next operation to be performed is a stop or start operation and upon outdoor air temperature.
Most control systems which control the temperature control systems of commercial buildings today have various energy management functions which the control systems can perform. For example, the setpoint temperature within the building can be set up during summer months and set back during winter months when the building is not occupied in order to reduce the cost of running the temperature control systems during times of non-occupancy. Together with the set up and set back function, the systems can perform an optimum start function in which the air conditioning system is reenergized for an amount of time calculated to bring the space temperature within the comfort range for the building by the start of occupancy. In addition, typical systems will also provide an optimum stop function in which the air conditioning system is shut off for a period of time prior to end of occupancy, this period of time designed to allow the space temperature to drift within the comfort range but not drift beyond the comfort range before the end of occupancy.
Prior optimum start and optimum stop systems of this nature have relied upon outdoor air temperature to determine the amount of lead time necessary in performing the optimum start/stop functions. For example, during the heating months, the colder the outdoor air temperature, the more lead time is necessary during an optimum start function and the less lead time is allowed during the optimum stop function to make sure that during the entire period of occupancy, the space temperature is within the desired comfort range. The comfort range is defined here as the range established between the minimum allowable comfortable space temperature and the maximum allowable comfortable space temperature. The use of outdoor air temperature by itself to determine the lead time for use in optimum start/stop functions is not ideal because each building, because of its construction and because of external factors other than outdoor temperature, such as wind and solar radiation, do not have the same heat loss for the same outdoor air temperature.
Thus, optimum start/stop functions have been developed to use space temperature drift rate in order to determine the lead time. The space temperature drift rate is determined in such systems by controlling the temperature control system at its off state for the optimum stop function and at its on state for the optimum start function and for then dividing the temperature swing while the air conditioning system is in this state by the amount of time that the temperature control system is in that state. For example, for an optimum start function and during winter months, the temperature control system is turned on at some time during the set back period and the temperature increase is then divided by the amount of time spanning that temperature increase in order to calculate drift rate which then can be used to determine the optimum start time. On the other hand, during winter months, the temperature control system can be turned off and the amount of temperature drift during that off time is divided by the amount of off time in order to determine the drift rate usable in optimum stop functions.
This approach, however, requires the temperature control system to be artificially turned on or off in order to determine drift rate usable in the optimum start/stop functions. Moreover, prior systems have tended to be inaccurate because they rely either upon only space temperature or only upon outdoor air temperature. The present invention overcomes many of these prior art failings by providing a system which utilizes temperature drift and dead time in order to determine the optimum lead time as modified by the outdoor air temperature.